Once someone agrees to speak, do not reply with only, “When are you free?” That leaves the other person to handle all the coordination. Instead, include several specific windows when you are available, along with your time zone.
When possible, offer options across at least two days and make the windows easy to scan. For example: “I am available Tuesday from 10:00 a.m. to noon ET, Wednesday from 2:00 to 5:00 p.m. ET, or Thursday after 4:00 p.m. ET.” Keep each window broad enough to accommodate a 15- to 20-minute conversation.